FIG. 1 diagrammatically illustrates, in cross-section, the configuration of a conventional buried Zener diode device 10 formed, for example, in a dielectrically isolated island region 12 of a semiconductor support substrate 14. (Typically, the topology of the Zener diode device is circular or conical so that the respective regions and surface contacts are generally conically or circularly shaped and are axially symmetrical with respect to an axis 16 that is orthogonal to the top surface 18 of the substrate.)
In particular, the device itself is shown as comprising a semiconductor island region 21 of lightly doped semiconductor material, such as P-type silicon, in a generally centrally located top surface portion of which a surface anode region 23 of heavily doped P- semiconductor material is formed. Surface anode region 23 has an generally annular or ring-shaped surface conductor 25 which provides the anode contact to the device. Adjacent to, but spaced apart from anode region 23 is an annular shaped deep N-type cathode region 31 which extends from surface 18 through island region 21 to an N+ underlying or buried region 33 at the bottom of the dielectrically isolated island. A Zener junction 34 is formed by a center column N-type region 35, which adjoins a portion of the bottom of anode region 23 and buried region 33. Annular shaped surface cathode region 31 has an generally annular or ring-shaped surface conductor 37 which provides the cathode contact to the device.
In a typical application as a voltage reference device, the anode and cathode terminals 25/37 of the Zener diode are coupled to a voltage monitoring circuit, which uses the Zener junction voltage for a specified control function. Because the voltage reference path to the Zener junction traverses a multiple region path through which current flows between the anode and cathode, what is measured by way of the anode and cathode contacts is necessarily inclusive of an accumulation of voltage drops across a serial resistance path through the device regions. The resistances of such regions which give rise to this series of voltage drops are schematically identified in FIG. 1 by resistance symbology associated with the respective regions and provided with same reference numerals as the regions, superseded by the letter R. For the cylindrically symmetric configuration shown, accordingly, there is a summed resistance value of (R31+R33+R35+R23) in series with the Zener junction voltage between any two anode/cathode contact points. Namely, the resistance R23 of the P+ semiconductor material of anode region 23 effectively masks the voltage at the true anode, while the accumulated value cf resistances R31+R33+R35 of the N type semiconductor material of regions 31, 33 and 35 effectively masks the voltage at the true cathode 36.
Although the individual resistances may be relatively small (e.g. on the order of 5-15 ohms), their values can vary widely due to variations in processing parameters or as a result of no control of sheet resistance during wafer processing, so that the total resistance of these regions may vary over a substantial range. This, coupled with differences in temperature coefficients, effectively prevents precision monitoring of the Zener junction voltage.